drm/i915: Fix system suspend without fbdev being initialized

In the Linux kernel, the following vulnerability has been resolved:

drm/i915: Fix system suspend without fbdev being initialized

If fbdev is not initialized for some reason - in practice on platforms
without display - suspending fbdev should be skipped during system
suspend, fix this up. While at it add an assert that suspending fbdev
only happens with the display present.

This fixes the following:

[ 91.227923] PM: suspend entry (s2idle)
[ 91.254598] Filesystems sync: 0.025 seconds
[ 91.270518] Freezing user space processes
[ 91.272266] Freezing user space processes completed (elapsed 0.001 seconds)
[ 91.272686] OOM killer disabled.
[ 91.272872] Freezing remaining freezable tasks
[ 91.274295] Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
[ 91.659622] BUG: kernel NULL pointer dereference, address: 00000000000001c8
[ 91.659981] #PF: supervisor write access in kernel mode
[ 91.660252] #PF: error_code(0x0002) - not-present page
[ 91.660511] PGD 0 P4D 0
[ 91.660647] Oops: 0002 [#1] PREEMPT SMP NOPTI
[ 91.660875] CPU: 4 PID: 917 Comm: bash Not tainted 6.2.0-rc7+ #54
[ 91.661185] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20221117gitfff6d81270b5-9.fc37 unknown
[ 91.661680] RIP: 0010:mutex_lock+0x19/0x30
[ 91.661914] Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 53 48 89 fb e8 62 d3 ff ff 31 c0 65 48 8b 14 25 00 15 03 00 <f0> 48 0f b1 13 75 06 5b c3 cc cc cc cc 48 89 df 5b eb b4 0f 1f 40
[ 91.662840] RSP: 0018:ffffa1e8011ffc08 EFLAGS: 00010246
[ 91.663087] RAX: 0000000000000000 RBX: 00000000000001c8 RCX: 0000000000000000
[ 91.663440] RDX: ffff8be455eb0000 RSI: 0000000000000001 RDI: 00000000000001c8
[ 91.663802] RBP: ffff8be459440000 R08: ffff8be459441f08 R09: ffffffff8e1432c0
[ 91.664167] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000001
[ 91.664532] R13: 00000000000001c8 R14: 0000000000000000 R15: ffff8be442f4fb20
[ 91.664905] FS: 00007f28ffc16740(0000) GS:ffff8be4bb900000(0000) knlGS:0000000000000000
[ 91.665334] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 91.665626] CR2: 00000000000001c8 CR3: 0000000114926006 CR4: 0000000000770ee0
[ 91.665988] PKRU: 55555554
[ 91.666131] Call Trace:
[ 91.666265] <TASK>
[ 91.666381] intel_fbdev_set_suspend+0x97/0x1b0 [i915]
[ 91.666738] i915_drm_suspend+0xb9/0x100 [i915]
[ 91.667029] pci_pm_suspend+0x78/0x170
[ 91.667234] ? __pfx_pci_pm_suspend+0x10/0x10
[ 91.667461] dpm_run_callback+0x47/0x150
[ 91.667673] __device_suspend+0x10a/0x4e0
[ 91.667880] dpm_suspend+0x134/0x270
[ 91.668069] dpm_suspend_start+0x79/0x80
[ 91.668272] suspend_devices_and_enter+0x11b/0x890
[ 91.668526] pm_suspend.cold+0x270/0x2fc
[ 91.668737] state_store+0x46/0x90
[ 91.668916] kernfs_fop_write_iter+0x11b/0x200
[ 91.669153] vfs_write+0x1e1/0x3a0
[ 91.669336] ksys_write+0x53/0xd0
[ 91.669510] do_syscall_64+0x58/0xc0
[ 91.669699] ? syscall_exit_to_user_mode_prepare+0x18e/0x1c0
[ 91.669980] ? syscall_exit_to_user_mode_prepare+0x18e/0x1c0
[ 91.670278] ? syscall_exit_to_user_mode+0x17/0x40
[ 91.670524] ? do_syscall_64+0x67/0xc0
[ 91.670717] ? __irq_exit_rcu+0x3d/0x140
[ 91.670931] entry_SYSCALL_64_after_hwframe+0x72/0xdc
[ 91.671202] RIP: 0033:0x7f28ffd14284

v2: CC stable. (Jani)

References: https://gitlab.freedesktop.org/drm/intel/-/issues/8015
(cherry picked from commit 9542d708409a41449e99c9a464deb5e062c4bee2)